Implantable lead and method of manufacture

ABSTRACT

An implantable, substantially isodiametric, low resistance implantable lead having at least one electrode positioned in a stimulation/sensing portion of the lead as well as a method of manufacturing the same. At least the stimulation/sensing portion is unitized through partially surrounding and supporting insulation and conductive element(s) of the stimulation/sensing portion with a fused matrix of material having mechanical properties consistent with a body of the lead.

RELATED APPLICATIONS

[0001] This is a division of U.S. patent application Ser. No.09/299,702, filed Apr. 26, 1999, pending.

FIELD OF THE INVENTION

[0002] The present invention relates to a lead, and in particular, to animplantable lead and a method of manufacturing such lead.

BACKGROUND OF THE INVENTION

[0003] Implantable leads having ring electrodes can be used in a varietyof applications, including delivery of electrical stimulation tosurrounding tissue, neural or otherwise, as well as measuring electricalenergy produced by such tissue. Whether serving in a stimulationcapacity or a sensing capacity, such leads are commonly implanted alongperipheral nerves, within the epidural or the intrathecal spaces of thespinal column, about the heart, and in the brain.

[0004] Notwithstanding the application, the common requirements for suchimplantable leads include flexibility, strength, and durability. Theextent of such qualities, of course, is dependent upon the nature of theuse, for example, temporary or permanent implantation. While materialselection and certain construction techniques can be tailored to assistin meeting these prescribed characteristics, an overriding considerationin the design of such leads is achieving at least an isodiametricstimulation/pacing portion thereof.

[0005] The benefits of achieving desired levels of flexibility,strength, and durability are intuitive. The isodiametric characteristicis likely less obvious. Depending upon the application, an isodiametriclead can reduce the potential for damage to the lead during insertion(for example, when a lead is passed through an insertion needle to reacha patient epidural space) and/or placement, improve the ability of thelead to pass through tissue or a vascular system, and is more resistantto being immobilized by tissue growth at a permanent implantation site.

[0006] Differing techniques have been used to produce isodiametricleads. One such technique concerns adhering a plurality of elements(i.e., conductive electrodes, conductive terminals, and spacinginsulative tubing material) to produce a generally integral body. Tubingmaterial separates a stimulation/sensing portion (i.e., alternatinginsulative tubing material and electrodes) from a terminal portion(i.e., alternating insulative tubing material and terminals). Theelectrodes, terminals, and tubing are independently formed but areintended to be isodiametric. Understandably, dimension variances in anyone element can result in a lead having a varying diameter.

[0007] Of further interest, to strengthen the plurality of elementinterfaces found in the stimulation/sensing portions and terminalportions of these leads, a composition, for example, medical gradeepoxy, is injected within an interior of the leads in and about thestimulation/sensing portions and the terminal portions. While thistechnique does typically effect stabilization and strengthening of thesecritical regions, the end result can also be that these regions are toorigid and even brittle.

[0008] Other techniques include applying a ring electrode(s) about anexterior surface of insulative tubing that forms the main body of thelead. The insulative tubing may be prepared to receive the electrode,for example, milled to remove an amount of material substantially equalto the material thickness of the ring electrode. Alternatively, theinsulative tubing may be unprepared, for example, a ring electrode issimply “crimped” to a diameter substantially equal to the otherwiseunadulterated diameter of the tubing.

[0009] For all of the methods described above, a finished lead is stillcomprised of a plurality of independent components brought together inan effort to form an isodiametric cross-section. Element misalignment,inaccuracies in grinding, variances in electrode material thickness orindividual element dimensions, or over/undercrimping could respectivelyresult in at least undesirable variances in lead diameter.

[0010] Accordingly, a need exists for a lead, as well as a method offabricating such lead, that provides a requisite level of flexibility,strength, and durability, while further providing a true isodiametricbody for at least the stimulation/sensing portion of the lead.

SUMMARY OF THE INVENTION

[0011] One aspect of the present invention is directed to an implantablelead including a lead body, having a distal end and a proximal end,whereas the lead body is formed of a material having prescribedmechanical properties. Extending from the distal end of the lead body, afirst region includes a plurality of electrodes. A first insulativematerial, having mechanical properties consistent with the material ofthe lead body, separates adjacent electrodes. Extending from theproximal end of the lead body, a second region includes at least oneterminal. A second insulative material, having mechanical propertiesconsistent with the material of the lead body, separates adjacentterminals. A conductor couples each terminal to at least onecorresponding electrode of the plurality of electrodes, wherein theconductor(s) extends along an interior passage defined by the lead body,first region, and second region. In addition to the at least oneconductor, the interior passage of the first region is substantiallyfilled with a third insulative material having mechanical propertiesconsistent with the material of the lead body.

[0012] Another aspect of the present invention concerns a method offorming a substantially isodiametric lead. Specifically, such lead has aprescribed diameter and includes at least one electrode separated fromat least one terminal by a lead body, wherein the at least one electrodeis electrically coupled to the at least one terminal by a conductorpassing through a passage defined by at least the lead body. The formingsteps include assembling the at least one electrode and the at least oneterminal relative to the lead body to form an assembly, includingconnecting the at least one electrode to the at least one terminal viathe conductor. The assembly is subjected to an over-molding process thatover molds the assembly with a first material to form an intermediateassembly. This first material is compatible with and has mechanicalproperties consistent with a material of the lead body. Ultimately, theintermediate assembly is processed to remove all material of theintermediate assembly in excess of the prescribed diameter.

[0013] An object of the present invention is to avoid the shortcomingsof known leads and manufacturing techniques for the same.

[0014] Another object of the present invention is to provide a method offorming a lead having a true isodiametric body for at least thestimulation/sensing portion of the lead.

[0015] Another object of the present invention is to provide a leadhaving a true isodiametric body for at least the stimulation/sensingportion of the lead.

[0016] Another object of the present invention is to provide a leadhaving a low resistance from a terminal to a coupled electrode to reduceenergy consumption during system operation.

[0017] Other aspects, objects, and advantages of the present inventionwill be apparent to those of ordinary skill in the art having referenceto the following Specification together with the provided drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In reference to the following figures, like reference numeralsand letters indicate corresponding elements:

[0019]FIG. 1 is a perspective view of a multi-electrode lead inaccordance with the present invention;

[0020]FIG. 2 is a plan view of another embodiment of a multi-electrodelead in accordance with the present invention;

[0021]FIG. 3 is a sectional view of the lead of FIG. 2, taken along lineIII-III;

[0022]FIG. 4 is a perspective view of a preferred conductor;

[0023]FIG. 5 is a plan view of an assembly of elements on a mandrel usedto form a lead in accordance with the present invention;

[0024]FIG. 6 is a sectional view of a transitional element;

[0025]FIG. 7 is a perspective view of an electrode spacer;

[0026]FIG. 8 is a perspective view of a terminal spacer;

[0027]FIG. 9 is a sectional view of a stylet guide;

[0028]FIG. 10 is a sectional view of a cap electrode; and

[0029]FIG. 11 is a schematic representation of one embodiment of anassembly fixture used to assemble a lead in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Various embodiments, including preferred embodiments, will now bedescribed in detail below with reference to the drawings.

[0031]FIG. 1 illustrates a preferred embodiment of multi-electrode lead10. While the leads illustrated and generally discussed here have eightelectrodes, lead 10 of the present invention may be constructed havingany number of electrodes (i.e., one or more).

[0032] Lead 10 includes a proximal end 12 and a distal end 14. Theproximal end 12 includes a plurality of electrically conductiveterminals 16, and the distal end 14 includes a plurality of electricallyconductive electrodes 18. While typically each terminal 16 iselectrically connected to a single electrode 18 via a conductor 20 (FIG.3), a terminal 16 can be connected to two or more electrodes 18.

[0033] Terminals 16 and electrodes 18 are preferably formed of anon-corrosive, highly conductive material. Examples of such materialinclude stainless steel, MP35N, platinum, and platinum alloys. In apreferred embodiment, terminals 16 and electrodes 18 are formed of aplatinum-iridium alloy.

[0034] Spanning between electrodes 18 of the distal end 14 and terminals16 of the proximal end 12, body 22 is formed from a medical grade,substantially inert material, for example, polyurethane, silicone, orthe like. While the specific material used for body 22 is not criticalto the present invention, body 22 must be non-reactive to theenvironment of the human body, provide a flexible and durable (i.e.,fatigue resistant) exterior structure for the components of lead 10, andinsulate adjacent terminals 16 and/or electrodes 18.

[0035] Serving as a sheath, body 22 substantially provides the exteriorstructure that contains the internalized elements of lead 10.Specifically, body 22 provides an enclosure for each conductor 20 thatconnects a terminal 16 with one or more electrodes 18. Each conductor 20is formed of a conductive material that exhibits the desired mechanicalproperties of low resistance, corrosion resistance, flexibility, andstrength. For consideration, however, it should be appreciated that inthe context of a multiple electrode lead 10, a plurality of conductors20 are required to fit within the interior of body 22. Accordingly, thecross-sectional area of each conductor 20 is restricted. As but oneexample, for a lead in accordance with the present invention that has anouter diameter of approximately 0.055 inches, conductor 20 could be onthe order of approximately 0.0065 inches.

[0036] While stranded bundles of stainless steel, MP35N, platinum,platinum-iridium alloy, drawn-brazed silver (DBS) or the like can beused, the preferred embodiment of conductors 20 utilizes wires formed ofmulti-strands of drawn-filled tubes (DFT), as illustrated in FIG. 4.Each strand is formed of a low resistance material 20 a and is encasedin a high strength material 20 b (preferably, metal). A selected numberof strands (seven, for this example) are wound and coated with aninsulative material 20 c. With regard to the operating environment ofthe present invention, insulative material 20 c protects the individualconductors 20 if body 22 were breached during use. Wire formed ofmulti-strands of drawn-filled tubes to form conductors 20, as discussedhere, is available from Temp-Flex Cable, Inc. (City, State).

[0037] In addition to providing the requisite strength, flexibility, andresistance to fatigue, conductors 20 formed of multi-strands ofdrawn-filled tubes, in accordance with the preferred embodiment, providea low resistance alternative to other conventional materials.Specifically, a stranded wire, or even coiled wire, of approximately 60cm and formed of MP35N or stainless steel or the like would have ameasured resistance in excess of 30 ohms. In contrast, for the samelength, a wire formed of multi-strands of drawn-filled tubes, asillustrated in FIG. 4, could have a resistance less than 4 ohms.Accordingly, in a preferred embodiment, each conductor 20, having alength equal to or less than 60 cm, has a resistance of less than 25ohms. In a more preferred embodiment, each conductor 20, having a lengthequal to or less than 60 cm, has a resistance equal to or less than 10ohms. In a most preferred embodiment, each conductor 20, having a lengthequal to or less than 60 cm, has a resistance of less than 4 ohms.

[0038] As an alternative embodiment, body 22 can further encompassstylet tubing 24 (FIG. 3). Stylet tubing 24 extends from the proximalend 12 to a point within a distal portion of lead 10; however, in apreferred embodiment, stylet tubing 24 extends to cap electrode 34. Incooperative reference to FIG. 2, stylet tubing 24 operatively receivesstylet 100 for purposes of allowing better control over lead 10 duringplacement.

[0039] LEAD ASSEMBLY

[0040] While the following discussion provides but one example of asequence of steps to form a lead similar to that illustrated in FIGS. 2and 3. One having ordinary skill in this art shall appreciate that thefollowing steps may be performed in a differing order or otherwiseinconsequentially modified to still yield the present invention.Consequently, such minor variations are still regarded as being withinthe scope of the present invention and should be construed in suchmanner.

[0041] Furthermore, for purposes of illustration, the following exampleincludes certain physical dimensions to illustrate the relationshipbetween elements as well as effects of differing processes. Accordingly,the provided physical dimensions are used merely for example and shallnot restrict the scope of the present invention.

[0042] The following illustrative example concerns the construction ofan eight electrode, epidural lead that accommodates a stylet. Oneskilled in the art shall appreciate, however, that a lead in accordancewith the present invention may have more than or less than eightelectrodes and/or have a larger or smaller diameter than the followingexample and remain within the scope of this disclosure.

[0043] In reference to FIG. 5, stylet tubing 24 is positioned overmandrel 150. Stylet tubing 24 has an outer diameter of approximately0.02 inches.

[0044] Depending on the quantity of conductors 20 required (e.g., forthis illustration, eight) and the size (i.e., diameter) of suchconductors 20, arranging and securing conductors 20 can be problematicwhen they are being arranged and secured about an element having thedimensions of stylet tubing 24.

[0045] While any number of techniques may be used to achieve sucharrangement of conductors 20 relative to stylet tubing 24, FIG. 11illustrates an example of a fixture 200 that can assist in this task.Specifically, fixture 200 includes first rotary clamp 202, iris 204,iris 206, second rotary clamp 208, and clamp 210. Rotary clamps 202 and208 each include a corresponding plurality of conductor clamps 203.While not required, it is preferred that the plurality of conductorclamps 203 of each rotary claim 202 and 208 be positioned within anarbitrary perimeter 205, whereas perimeter 205 should be equal to orgreater than a fully-opened inner diameter of either iris 204 or 206.

[0046] As illustrated, mandrel 150, including stylet tubing 24, passesthrough irises 204 and 206 and second rotary clamp 208 and is securedbetween clamps 202 and 210. Each conductor 20 similarly passes throughirises 204 and 206 and is secured between respective clamps 203 ofrotary clamps 202 and 208.

[0047] Conductors 20 secured within fixture 200 are prepared forassembly in that a prescribed amount of insulative material 20 c isremoved at or about the proximal and distal ends of each conductor 20 toexpose conductive material 20 a and 20 b. As will be discussed later,this exposed conductive material 20 a and 20 b of the proximal anddistal ends of each conductor 20 is eventually joined to an electrode 18and a terminal 16. Accordingly, the exposed conductive material 20 a and20 b is arranged at differing positions relative to stylet tubing 24 toaccommodate the serial arrangement of terminals 16 and electrodes 18.

[0048] The rotational nature of rotary clamps 202 and 208 providesunobstructed access to the in-process lead 10. Specifically, uponsecuring a single conductor 20 between opposing (or non-opposing) clamps203, the rotary clamps 202 and 210 are simply rotated to allow access tounoccupied clamps 203.

[0049] When all of the conductors 20 are strung between claims 202 and208, irises 204 and 206 are actuated to close and draw conductor(s) 20closely about the outer diameter of stylet tubing 24. When conductor(s)20 are resting against the outer diameter of stylet tubing 24,conductor(s) 20 are secured in place. Conductor(s) 20 may be securedusing adhesive and/or subjected to a force applied through use of atemporary or permanent restraint, for example, one or more crimpedcollars.

[0050] While the illustration of FIG. 11 shows but one embodiment offixture 200, one skilled in the art should appreciate that othertechniques/structures may be employed to position conductors 20 adjacentan exterior surface of stylet tubing 24. Specifically, clamps 203 ofeach rotary clamp 202 and 208 could be moveable along respective radialpaths (not shown) that would allow strung conductors 20 to be moved froma first position to a second position adjacent the exterior surface ofstylet tubing 24. Alternatively, conductors 20 could initially besecured to one end of stylet tubing 24 and only a single iris could beused to draw the unsecured portions of conductors 20 toward stylettubing 24. As yet another alternative, while the various alternativesoffered provide some mechanism to control the rate of movement andrelative positioning of conductors 20, an operator could simplymanipulate the conductor(s) 20 to manually position and secure themrelative to stylet tubing 24.

[0051] Once all conductors 20 are secured to stylet tubing 24,transitional element 26, electrode(s) 18, electrode spacer(s) 28, outertubing 23, terminal spacer(s) 30, terminal(s) 16, and stylet guide 32are positioned over, and concentrically arranged with, stylet tubing 24.The arrangement of these elements is in accordance with that illustratedin FIG. 5.

[0052] Transitional element 26 is illustrated in FIG. 6. As will bediscussed later, transitional element 26 provides a platform to receivecap electrode 34 (FIG. 10). Transitional element 26 further provides adurable guide 26 a to direct a distal end (not shown) of stylet 100 tocap electrode 34 via passage 26 b. Transitional element 26 is preferablyformed of a conductive material, for example, the same material used toform electrodes 18.

[0053] Electrode spacer 28 is illustrated in FIG. 7. Similarly, terminalspacer 30 is illustrated in FIG. 8. Functionally, electrode spacer 28and terminal spacer 30 accurately defines a space between adjacentelectrodes 18 and terminals 16, respectively. Electrode spacer 28 andterminal spacer 30 are preferably formed of the same material as outertubing 23. However, spacers 28 and 30 may be formed of a material thatdiffers from that of outer tubing 23; provided however, any differingmaterial used for electrode spacer 28 and/or terminal spacer 30 must becompatible with and possess largely the same mechanical properties(e.g., non-reactive to the environment of the human body, flexible anddurable) as outer tubing 23. At least for purposes of this example,spacers 28 and 30 are formed of a polyurethane material, for example,Bionate 75D (Polymer Tech. Group, City, State). As is noted in FIG. 5,spacers 28 and 30 should have an outer diameter greater than lead 10.

[0054] Outer tubing 23 separates electrodes 18 from terminals 16. In apreferred embodiment, outer tubing 23 has a diameter substantially equalto a diameter of lead 10. Alternatively, outer tubing 23 may have adiameter less than lead 10, or a diameter greater than lead 10. Inregard to the latter alternative, outer tubing 23 must have a wallthickness greater than a differential between a radius of lead 10 and aradius (to the outer diameter) of outer tubing 23. For this particularexample, outer tubing 23 has a nominal outer diameter of approximate0.055 inches.

[0055] Stylet guide 32 is illustrated in FIG. 9. Stylet guide 32provides an inlet to stylet tubing 24. Stylet guide 32 is preferablyformed of a conductive material, for example, the same material used toform electrodes 18. Stylet guide 32, as well as terminals 16, electrodes18, and transitional element 26, preferably each have an outer diameterequal to or greater than a nominal diameter of lead 10. In a morepreferred embodiment, these elements each have an outer diameter greaterthan a nominal diameter of lead 10.

[0056] Following the assembly of each of the elements described above,terminals 16 and electrodes 18 are joined to their respective conductors20. Generally, each terminal 16 (and each electrode 18) is positionedrelative to exposed conductive material 20 a and 20 b of a conductor 20and is joined in a manner that facilitates a transfer of electricalenergy, for example, resistance weld or laser weld. Once all terminals16 and electrodes 18 are secured, stylet guide 32 is secured to aproximal-most terminal 16, and transitional element 26 is secured to adistal-most electrode 18. Provided transitional element 26 and styletguide 32 are formed a conductive material, these elements may be securedusing a process consistent with that used to join terminals 16 andelectrodes 18 with conductors 20. Otherwise, transitional element 26 andstylet guide 32 can be joined using an adhesive, cement or the like.

[0057] The completed assembly (FIG. 5) is then over-molded, using wellknown injection molding techniques, using a material having mechanicalproperties consistent with a material(s) used to form outer tubing 23,electrode spacer 28, and terminal spacer 30. In a preferred embodiment,the over-molding material and the material of outer tubing 23, electrodespacer 28, and terminal 28 are the same.

[0058] This process has the beneficial effect of unitizing the elementassembly to form lead 10. Moreover, electrode spacers 28 and terminalspacers 30 are placed in a state of flow, which, at least in part,results in a filling of regions between terminals 16/electrodes 18 andstylet guide 24. Consequently, terminals 16 and electrodes 18 arepartially surrounded (i.e., along an interior surface) and supported bya fused matrix of material. Importantly, as electrode spacers 28 andterminal spacers 30 are formed of a material mechanically equivalent tothat of body 22/outer tubing 23, the stimulation/sensing portion andterminal portion of lead 10 are stabilized and strengthened while alsoretaining their flexible properties.

[0059] The over-molded assembly (not shown) is then subjected to agrinding process to remove all excess material. In a preferred process,the over-molded assembly is subject to centerless grinding, whereinexcessive material, including over-molded material, electrode material,terminal material, and the like, is removed. Pursuant to the describedover-molding and grinding of the entire lead assembly, an isodiametriclead is obtained, which is further free of any gaps or voids betweeninsulative material and conductive material that may otherwise exist inconventional devices.

[0060] Following the grinding process, cap electrode 34 is affixed totransitional element 26 using conventional means, for example,resistance welding, laser welding, or the like.

[0061] While addressed in part above, as the invention has beendescribed herein relative to a number of particularized embodiments, itis understood that modifications of, and alternatives to, theseembodiments, such modifications and alternatives realizing theadvantages and benefits of this invention, will be apparent to those ofordinary skill in the art having reference to this specification and itsdrawings. It is contemplated that such modifications and alternativesare within the scope of this invention as subsequently claimed herein,and it is intended that the scope of this invention claimed herein belimited only by the broadest interpretation of the appended claims towhich the inventors are legally entitled.

What is claimed is:
 1. An implantable lead comprising: a lead bodyhaving a distal end and a proximal end; an electrode positioned at thedistal end of the lead body; a terminal positioned at the proximal endof the lead body; and a conductor, electrically coupling the electrodeand the terminal and extending along an interior passage defined by thelead body, wherein the conductor has a resistance equal to or less than25 ohms for a conductor length equal to or less than 60 cm.
 2. Animplantable lead in accordance with claim 1 , wherein the conductor isformed of stranded wire.
 3. An implantable lead in accordance with claim1 , wherein an outer diameter of the lead body is approximately 0.05inches.
 4. An implantable lead in accordance with claim 1 , furthercomprising a stylet guide, positioned within the interior passagedefined by the lead body, wherein an inlet of the stylet guide is at theproximal end of the lead body.
 5. An implantable lead in accordance withclaim 1 , wherein the implantable lead is substantially isodiametric. 6.A method for forming a substantially isodiametric lead having aprescribed diameter and at least one electrode separated from at leastone terminal by a lead body, wherein the at least one electrode iselectrically coupled to the at least one terminal by a conductor passingthrough a passage defined by the lead body, comprising the steps of:assembling the at least one electrode and the at least one terminalrelative to the lead body to form an assembly, including connecting theat least one electrode to the at least one terminal via the conductor;over-molding the assembly with a first material to form an intermediateassembly, wherein the first material is compatible with and hasmechanical properties consistent with a material of the lead body; andremoving all material of the intermediate assembly in excess of theprescribed diameter.
 7. A method in accordance with claim 6 , whereinthe at least one electrode has an outer diameter greater than theprescribed diameter prior to the removing step.
 8. A method inaccordance with claim 6 , wherein the at least one terminal has an outerdiameter greater than the prescribed diameter prior to the removingstep.
 9. A method in accordance with claim 6 , wherein the removing stepinvolves subjecting the intermediate assembly to at least a centerlessgrinding process.
 10. A method for forming a substantially isodiametriclead having a prescribed diameter and a first region separated from asecond region by a lead body, the first region having a plurality ofelectrodes, and the second region having a plurality of terminals, eachterminal being respectively and electrically joined to at least oneelectrode by a conductor passing through a passage defined by the firstregion, second region, and lead body, comprising the steps of:assembling the plurality of electrodes and plurality of terminalsrelative to the lead body to form an assembly, this step includingelectrically coupling each terminal to at least one electrode by aconductor; and unitizing at least that portion of the assemblycorresponding to the first region of the lead, wherein subsequent tounitization, each electrode is separated by an insulative material, andthe passage defined by at least the first region is substantially filledwith the insulative material.
 11. A method in accordance with claim 10 ,further comprising unitizing that portion of the assembly correspondingto the second region of the lead, wherein subsequent to unitization,each terminal is separated by a second insulative material, and thepassage defined by at least the first region is substantially filledwith the second insulative material.
 12. A method in accordance withclaim 11 , wherein the second insulative material has mechanicalproperties consistent with the material of the lead body.
 13. A methodin accordance with claim 10 , wherein the insulative material hasmechanical properties consistent with the material of the lead body. 14.A method for forming a substantially isodiametric lead having aprescribed diameter and a first region separated from a second region bya lead body, the first region having a plurality of electrodes, and thesecond region having a plurality of terminals, each terminal beingrespectively and electrically joined to at least one electrode by aconductor passing through a passage defined by the first region, secondregion, and lead body, comprising the steps of: assembling the pluralityof electrodes and plurality of terminals relative to the lead body toform an assembly, this step including electrically coupling eachterminal to at least one electrode by a conductor; and unitizing atleast that portion of the assembly corresponding to the first region ofthe lead, wherein each electrode is separated by an insulative material,and the passage defined by at least the first region is substantiallyfilled with the insulative material, wherein the step of unitizinginvolves over-molding the assembly with a second material to form anintermediate assembly, wherein the second material is compatible withand has mechanical properties consistent with the material of the leadbody.
 15. A method in accordance with claim 14 , wherein the secondmaterial and the insulative material are the same.
 16. A method inaccordance with claim 14 , further comprising the step of removing allmaterial of the intermediate assembly in excess of the prescribeddiameter.
 17. A method in accordance with claim 16 , wherein the step ofremoving involves subjecting the intermediate assembly to at least acenterless grinding process.
 18. A method in accordance with claim 16 ,wherein the at least one electrode has an outer diameter greater thanthe prescribed diameter prior to the removing step.
 19. A method inaccordance with claim 16 , wherein the at least one terminal has anouter diameter greater than the prescribed diameter prior to theremoving step.